High strength intended for ensuring collision safety performance, and fuel economy due to reduction in car body weight is required of a steel sheet for use in automobile framework components, and so forth. Further, excellent formability is required of the steel sheet in order to work it into the automobile framework components that are complex in shape.
For this reason, a high-strength steel sheet having not only tensile strength (TS) on the order of 980 MPa or more but also stretch flangeability (a hole expanding ratio: λ) more enhanced than in the case of conventional steel, and a high-strength steel sheet more enhanced not only in stretch flangeability but also in total elongation (total elongation: El) have been highly desired. Further, in an application sector where stretch flangeability is anticipated to exhibit a particularly excellent effect although elongation performance is the same as in the past, a hole expanding ratio 125% or higher is desired of a high-strength steel sheet with tensile strength (TS) on the order of 980 MPa or more. Furthermore, in an application sector where enhanced performance in both the elongation, and the stretch flangeability is desired, the total elongation 13% or more, and the hole expanding ratio 90% or higher are desired of the high-strength steel sheet with tensile strength (TS) on the order of 980 MPa or more.
Further, materials designing on the basis of tensile strength (TS) has thus far been adopted, however, since it has become important to make an assessment on yield strength (YP) when collision safety is taken in consideration, a high-strength steel sheet excellent in both yield strength, and formability is now in demand. As specific mechanical characteristics of the high-strength steel sheet described as above, there desired yield strength (YP) 900 MPa or higher, total elongation (El) 10% or more, and stretch flangeability (a hole expanding ratio: λ) 90% or more, or preferably 100% or more, are desired.
In consideration of such needs as described, and on the basis of various ideas for structure control, there have been proposed a multitude of high-strength steel sheets with improvement in stretch flangeability, or balance between elongation and stretch flangeability. However, a high-strength steel sheet satisfying such desired levels as above has not been completed as yet at the present stage.
For example, in Patent Document 1, there is disclosed a high tensile-strength cold-rolled steel sheet comprising at least one element selected from the group consisting of Mn, Cr, and Mo, in total content of 1.6 to 2.5 mass %, effectively composed of a single-phase structure of martensite. With this high tensile-strength cold-rolled steel sheet, a hole expanding ratio (stretch flangeability) 100% or more is obtained while ensuring tensile-strength 980 MPa or more, but the hole expanding ratio has not reached 125% as yet, and elongation is yet to reach 10%.
In Patent Document 2, there is disclosed a high tensile-strength steel sheet composed of a dual-phase structure of ferrite 65 to 85% in area ratio, and tempered martensite in the balance. With this steel sheet, since the area ratio of ferrite is excessively high, the hole expanding ratio has not reached 90% although elongation 13% or more has been obtained.
Further, in Patent Document 3, there is disclosed a high tensile-strength steel sheet composed of a dual-phase structure wherein ferrite, and martensite each have an average grain size 2 μm or less, and martensite has a volume ratio in a range of 20 to 60%, however, the hole expanding ratio thereof is less than 90%.
Further, it is well known that, besides the constituents of a matrix structure itself, as set forth in each of Patent Documents 1 to 3, described as above, inclusions (sulfide, in particular) present in the matrix structure, as well, have significant effects on the stretch flangeability.
For example, in Non-patent Document 1, it is disclosed that, in the case of a steel sheet having tensile strength (TS) on the order of 440 to 590 MPa, reduction in the sulfur content of the steel sheet can suppress generation of inclusions, thereby improving stretch flangeability.
However, in order to reduce the sulfur content of the steel sheet down to a level lower than the present level, there will be the need for a special desulfurization treatment to be applied in a steel-making process, thereby causing deterioration in productivity, and an increase in production cost. Therefore, techniques for improvement on stretch flangeability by reduction in the sulfur content, as disclosed in Non-patent Document 1, will be difficult for application on an industrial basis.
In Patent Document 4, there is disclosed a high yield-strength and high tensile-strength cold-rolled steel sheet excellent in formability, characterized in that a steel sheet comprising C: 0.02 mass % or less, and Ti: in a range of 0.15 to 0.40 mass % is subjected to annealing at a temperature in a range of 600 to 720° C. in a carburizing atmosphere. With this steel sheet, yield-strength 900 MPa or higher, and elongation 10% or more have been obtained, but stretch flangeability is less than 90%.    [Patent Document 1] JP-A-2002-161336    [Patent Document 2] JP-A-2004-256872    [Patent Document 3] JP-A-2004-232022    [Patent Document 4] JP-A-2007-9253    [Non-patent Document 1] “NKK Technical Report”, published by Nippon Koukan K. K., by Masayuki Kinoshita, et al., Vol. 145, 1994, p. 1